The present invention relates to a received signal processing apparatus for processing with a majority principle a repetitive digital signal which is transmitted from a transmit station by radio equipment and representative of the same information.
In communications art, there is known an error correction system in which while a transmit station transmits the same information repeatedly, a receive station corrects errors on a decision-by-majority basis so as to enhance reliability of information. This kind of system is simpler than the other known error correction systems and extensively used. In the system employing the majority principle, how to detect errors is the important problem. Some approaches have heretofore been proposed for the detection of errors, e.g., one which determines whether the same pattern repeatedly occurs and, if it does, processes a received signal as a correct signal, and one which determines a field intensity of a received signal and, if it is at a sufficient level, processes it on a majority basis to determine the transmission quality of the signal.
The problem with the repetitive pattern scheme is that where the error probability is very high such as where the bit error rate is 10.sup.-2 or higher, and where a single information pattern has 50 to 100 bits, for example, the probability that the received signal is decided correct is sharply reduced and, instead, the probability that no signal is detected, i.e., the non-detection probability is increased. On the other hand, the field intensity scheme fails to provide satisfactory information on the deterioration of signals partly because upon appearance of an interference wave the system decides that the field intensity is sufficient and, therefore, fails to detect an error which actually exists in a signal, and partly because depending upon temperature and other ambient conditions the measured field intensity information and the bit error rate of received signal do not always constantly correspond to each other.